Methods and systems for measuring distance are used in many applications. Examples for this include very precise measurements in geodesic applications, but also measurement problems in the field of building installation or for industrial process controls. In these problems, use is made of stationary, movable or else portable distance measuring devices which carry out an optical distance measurement to a selected measurement point. Here, a laser beam is usually emitted and received again, and evaluated, after reflection at the target. Here, different measurement principles are available for determining the distance, for example a phase measurement or a time-of-flight measurement.
Particularly in the field of building installation or building inspection, use is made of portable and handheld devices which are placed in relation to a structure to be measured and then carry out a distance measurement to a surface. Portable distance measuring devices which are suitable and typical for such applications are described in e.g. EP 0 738 899 and EP 0 701 702. Since a measurement point visible on the surface to be measured is advantageous for most applications, red lasers are usually used as radiation sources for distance measurements. In conjunction with great ease of handling, accuracies down to the millimeters range can be achieved with rangefinders in the prior art. Using conventional portable distance measuring devices, it is possible to carry out measurements from one point to another point to which there is a sight connection. If the target is concealed, many devices may also ascertain horizontal mass by means of an inclination sensor.
One possibility for determining a distance between two points, which can also be used if there is no line of sight between the points, is calculation by means of trigonometry. This is already known sufficiently from ground-based surveying devices, such as theodolites or total stations. For trigonometrically ascertaining a distance a between two spatial points B and C, it suffices to know the distance to these two points from a third point A, and the angle α at point A between the sides b and c in the direction of the points B and C. The length of a can then be calculated by means of the cosine law:a=√{square root over (b2+c2−2·b·c·cos α)}
Although a conventional handheld distance measuring device from the prior art makes it possible to measure the distances b and c to the spatial points B and C exactly, a function for accurately and reliably determining the angle α is usually missing. Currently available acceleration sensors cannot yield a sufficiently reliable value for a for distance calculation purposes, and compasses are susceptible to disturbance particularly in interiors of buildings; at best angles in the vertical can be ascertained with sufficient accuracy and reliability by means of inclination sensors.
The prior art describes various solutions with portable distance measuring devices comprising laser rangefinders by means of which two points can be targeted simultaneously or sequentially, wherein an angle between the emission directions of the two lasers can be determined.
EP 2 698 602 A1 discloses such a distance measuring device with a referencing unit that may be folded out, the latter being brought into contact with a surface, wherein a solid angle between a first alignment and a second alignment of the device relative to the surface may be determined by means of rotary encoders. On the one hand, this approach is structurally complex and, on the other hand, requires the presence of a suitable surface.
Document EP 1 664 674 B1 relates to a method and a system for determining the actual position of a portable measuring device in space. To this end, the measuring device additionally comprises an active scanning functionality for scanning the space by means of a laser beam and for capturing a plurality of reflectors distributed in the space. This solution is also structurally complex.
Furthermore, it requires a relatively time-consuming distribution and attachment of a multiplicity of reflectors in space and the recollection thereof after completing the measurements.
Camera-based optical methods require less structural outlay. Such methods known from the prior art use either image stitching or SLAM (simultaneous localization and mapping) for ascertaining the change in pose. Thus, disclosed in the application EP 2 669 707 A1 discloses a portable distance measuring device for carrying out a method for indirectly determining distances by means of two directly measured distances and an angle, the angle being ascertained from a panoramic image which was generated by means of image stitching from images recorded in the direction of the distance measurement by a camera of the distance measuring device. By contrast, in the method described in WO 2015/073548 A2, an object of known dimensions is recorded in a first image in order to obtain a scale for the recorded image. A change in the pose of the device is then ascertained by means of SLAM. However, on the one hand, these methods place certain requirements on the recorded surface, which must have a sufficient number of features which may be uniquely captured by optical means, for image stitching or SLAM. In particular, these methods cannot be sensibly employed in the case of measurements on unstructured (e.g. uniformly white) walls. On the other hand, a significant computational outlay also arises disadvantageously.